In the course of manufacturing paper and similar products, including paperboard and the like, it is well-known to incorporate quantities of inorganic materials into the fibrous web in order to improve the quality of the resulting product. In the absence of such fillers, the resultant paper can have a relatively poor texture due to discontinuities in the fibrous web. The said fillers are also important in improving the printing qualities of the paper, i.e., by improving the surface characteristics of same. The use of appropriate such fillers further vastly improves the opacity and the brightness of a paper sheet of a given weight. The brightness and opacifying sheet, one which incorporates whiteness, high opacity, good printability and light weight.
According to Koppelman et al. in U.S. Pat. No. 4,650,521, it has become customary in the industry to beneficiate the crude kaolinitic clays used in clay filler compositions for paper making to improve particle size distribution and also improve color characteristics by removing ferric iron-containing compounds in the clay. Such ferric iron-containing compounds impart a non-white color to the clay and reduce the overall brightness or reflectance to visible light of the clay. It is well-known that the effect of these ferric iron-containing compounds may be reduced by treating the clay with a reducing agent which converts the ferric ion to the less highly colored ferrous ion. A variety of reducing agents are known to be suitable for treating kaolinitic clays, but the most commonly used reducing agents are water-soluble dithionites or sulphites, such as sodium dithionite, zinc dithionite, sodium bisulphite, sodium hydrosulphite, and sodium pyrosulphite. Further, according to the patentees, in the conventional process for reducing the ferric iron-containing impurities in a kaolinitic clay to the ferrous state, a low solids aqueous suspension of the crude clay is first formed, then if desired, degritted to remove large particles, and then treated with a reducing agent to convert the ferric ions therein to the ferrous state. The ferrous ion is generally very soluble in water and will pass into the water in which the clay is suspended. The treated clay is then thickened, dewatered by filtration and the resultant filter cake thermally dried to produce a clay filler product having a high solids content suitable for economic transport. Such a low solid content process requires that the clay suspension be in a fluid state, that is, that the solids content of the crude clay suspension be less than about 50% by weight and usually in the range of 20% to 35% by weight. Unfortunately such low solids processing of the crude kaolin requires that significant dewatering and drying be carried out to ready the treated clay product for economic transport. Significant economic benefits would be obtained if the crude kaolin clay could be processed at a high solids content so that the dewatering and subsequent drying of the treated clay could be minimized or eliminated.
These are also some of the objectives of the present invention but simplified measures and lower work input are used for their attainment.
Thus, an object of the invention is to produce a bleached, dispersed, high solids slurry from unprocessed crudes at low pH, and with minimum work input, and to make the slurry at the highest solids possible without filtration or spray drying.
A further object is the production of fine coating clay slurries for paper using fine clays such as the South Carolina clays.